Forty-year-old telescope is still a galactic explorer

Most 40-year-old electronic systems have long since been consigned to the scrap heap. But astronomers who use the giant 305-metre Arecibo radio telescope in Puerto Rico say it still offers unique capabilities, despite a review panel that last year urged the National Science Foundation to cut its support. The radio dish - the largest in the world - is needed to test general relativity and theories of galactic evolution, its supporters say.

"We're very hopeful about the future," says Bob Brown, director of the National Astronomy and Ionosphere Center, which operates Arecibo. Brown was speaking after a recent meeting in Washington, DC, US, discussing the cutting-edge astronomy that can still be done with the observatory.

Although not movable, Arecibo's biggest strength is its size - it is three times wider than the world's second-biggest dish, the steerable 100-metre Robert C Byrd Green Bank Telescope in West Virginia. Its size makes it extremely sensitive to radio waves at any given instant in time, allowing it to detect speedy changes in faint objects, or compare many points across the sky.

Testing Einstein

That sensitivity keeps it in demand - astronomers submit requests for three to four times more observing time than is available. When the centre of the galaxy comes into its field of view, that demand climbs to nine times more time than is available.

Arecibo is famed historically for discovering many pulsars. Now astronomers want to take advantage of its high sensitivity to detect changes in the timing of pulses from these dense, spinning stars.

Such observations could reveal how the stars are rippling the fabric of space-time around them by emitting gravitational waves as they rotate - providing a test of Einstein's theory of general relativity, which predicts the phenomenon. "That kind of science is unique to Arecibo," and important to physics as a whole, says Brown.

The Washington meeting also discussed plans to map the distribution of intergalactic hydrogen across the sky. Earlier surveys sought to measure the amount of hydrogen linked to nearby galaxies, but the new survey will look in areas where no galaxies are present.

Asteroid hunt

The goal is to see if large amounts of unconsolidated hydrogen gas are primordial remnants of the Big Bang, or were stripped from galaxies that were interacting with each other as they evolved. The answer will have a big impact on astronomers' understanding of galactic evolution, Brown says.

Arecibo can observe asteroids at twice the distance of the only other planetary radar, on NASA's 70-metre Goldstone antenna in California, US, although Goldstone can observe more of the sky because it is fully steerable.

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The 305-metre Arecibo dish is the world's most sensitive radio telescope (Image: NAIC/Arecibo Obs/NSF)